Occupant protecting device for use in motor vehicles

ABSTRACT

An occupant protecting device for a motor vehicle, which inflates an air-bag system reliably to protect occupants in the event of a collision with high reliability. The occupant protecting device includes an acceleration sensor for sensing an acceleration signal of a vehicle in the event of collision with another vehicle or the like, a signal processor for detecting a collision waveform from an acceleration signal output from the acceleration sensor, a comparator for determining whether or not the output signal of the signal processor exceeds a preset level to produce a trigger signal when it exceeds the preset level, a latch circuit for latching the trigger signal from the comparator, and a firing circuit for operating an occupant protecting unit in response to a drive signal from the latch circuit.

This is a divisional of application Ser. No. 08/063,752, filed May 20,1993 now U.S. Pat. No. 5,506,775.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an occupant protecting device for usein motor vehicles for protecting occupants sitting on seats of a motorvehicle in the event of a collision, and more particularly to anoccupant protecting device for a motor vehicle, which inflates anair-bag system reliably to protect occupants in the event of acollision.

2. Description of the Related Art

FIG. 1 is a block diagram showing an example of a conventional occupantprotecting device for use in motor vehicles.

In the figure, an acceleration sensor 1 senses an acceleration signal ofa vehicle which is generated in the event of a collision with anothervehicle or the like. A signal processor 2 includes an integrator forintegrating the acceleration signal output from the accelerationsensor 1. The acceleration signal shows a waveform representing thecollision. A comparator 3 determines whether or not the output signal ofthe signal processor 2 exceeds a preset level so as to produce a triggersignal when it exceeds the preset level. A one-shot multi-vibrator ordrive circuit 4 latches a trigger signal from the comparator 3 for apreset period of time, and continuously produces a drive signal duringthe preset period. An ignition device 5 which forms a primary portion ofthe occupant protecting device, operates to fire in response to a drivesignal from the one-shot multi-vibrator 4. When the ignition device 5 istriggered to inflate an air bag or bags and/or to strain a seat belt orbelts.

In the occupant protecting device thus constructed which utilizes theintegration value output from the signal processor 2, the comparator 3judges whether or not the collision is dangerous to occupants, on thebasis of the variation with time. When it is dangerous, the comparator 3generates a trigger signal which in turn triggers the drive circuit 4 asthe one-shot multi-vibrator.

Therefore, the trigger signal, the pulse width of which is notsufficiently wide, fails to trigger the one-shot multivibrator 4. Theoccupant protecting device may be unable to protect the occupants insuch a collisional accident.

Next, a power source circuit suitable for the occupant protectingdevices including an air bag, for example, thus far described will bedescribed. In the occupant protecting device, when the power line isaccidentally disconnected, a power supply is interrupted. In theaccident, the energy stored in the capacitor contained in the outputside of the device must be effectively used, and the power sourcecircuit must be reliable.

FIG. 8 is a circuit diagram showing a conventional power source circuitfor an igniting device of a vehicle occupant protecting device.

In FIG. 8, reference numeral 61 designates a battery; 62, an ignitionswitch; 63, a controller; and 64, a DC power source for air-baginflation.

In the DC power source 64, the battery voltage which is supplied fromthe battery 61 through the ignition switch 62 to the controller 63, isboosted by a DC--DC converter 65 to be applied through a resistor 66 toan output capacitor 67. As a result, the output capacitor 67 is chargedwith the boosted voltage.

The output capacitor 67 has a large capacity since it must supply apower to a squib 80.

In the circuit, block diodes 68a and 68b are provided to block a reversecurrent, and a diode 69 is provided to block the rush current into theoutput capacitor 67.

A diagnosis circuit 70 includes a CPU. A diagnosis power source 71receives a voltage from a car-carried battery 61, and supplies arequired power to the diagnosis circuit 70.

A backup capacitor 71a is connected to the input side of the diagnosispower source 71. The backup capacitor 71a has a smaller capacity thanthe output capacitor 67.

A storage portion 72 transfers data to and from the diagnosis circuit70.

Collision detecting units 74 to 79 are mounted at required parts of acar body. Each collision detecting unit includes an acceleration switchfor closing in response to a predetermined change of acceleration, and aresistor. The collision detecting unit 74, for example, is made up of anacceleration switch 74a and a resistor 74b.

A squib 80, mounted on a steering portion, serves as an ignitingelectrode for igniting a powder to inflate an air bag (not shown)provided in the steering portion.

A spiral cable 81, which is a flexible code wound around the steeringshaft, connects the squib 80 to a controller 63 electrically. The spiralcable 81 and the squib 80 constitute an operation controller 80A.

In the figure, only one squib 80 is illustrated, but if required, aplural number of squibs 80 may be provided at other seats. In this case,those squibs are electrically arranged in parallel.

Reference numerals 73a to 73n designate output terminals of thecontroller 63. The output terminal 73a is connected to a line L1,through the collision detecting unit 74, output terminal 73b, outputterminal 73e, spiral cable 81, squib 80, and output terminal 73f.

The output terminals 73h, 73j, 73l, and 73n are connected to a line L2.

The collision detecting unit 75 is connected between the output terminal73a, 73c and 73d.

Similarly, the collision detecting units 76, 77, 78, and 79 arerespectively provided between the output terminals 73g and 73h, theoutput terminals 73i and 73j, the output terminals 73k and 83, and theoutput terminals 73m and 73n.

The output terminals 73h, 73j, 73l, and 73n are connected to the lineL2, which is grounded.

The diagnosis circuit 70, contained in the controller 63, is groundedthrough a connector harness 82 and a switch SW, and also through aconnector harness 83 and an alarm lamp La.

The operation of the power source circuit thus arranged will bedescribed.

When the ignition switch 62 is closed, the voltage supplied from thebattery 61 is boosted by the DC--DC converter 65. The boosted voltage isapplied through the diode 68c to the output capacitor 67. The capacitor67 is charged with a time constant which is determined by the resistor66 and the output capacitor 67. The voltage applied to the outputcapacitor 67 is always higher than the battery voltage from the battery61.

Any of the acceleration switches 74a to 79a is placed to a stateresembling a short-circuited state, and a voltage across thecorresponding resistor of those resistors 74b to 79b takes a normalvalue, the diagnosis circuit 70 determines whether the cause bringingabout such a state of the switch is the failure or the collision, andstores the result of the determination into the storage portion 72.

The storage portion 72 stores information on which of the accelerationswitches 74a to 79a is turned on by the collision.

In the event of a collision with another vehicle, for example, anegative acceleration is generated, as a result of which any of theacceleration switches 74a and 75b and any of the acceleration switches76a to 79a is turned on. The charge derived from the output capacitor 67flows through a route as indicated by a broken line. The squib 80 isheated, the powder is fired, and the air bag is inflated.

A serious collisional accident occurs, and a large impact is applied tothe diagnosis circuit 70. Even under such severe conditions, thediagnosis circuit 70 must operate normally in order that it can grasp astate of the air bag system correctly.

In the conventional power source circuit arranged described above, ifthe ignition switch 62 is disconnected from the controller 63 by thecollision, the power supply to the diagnosis circuit 70 is interrupted.As a result, the diagnosis circuit 70 will be stopped in operation.

The diagnosis circuit 70 is not stopped at the instant that thedisconnection occurs since the electrical energy is supplied from thebackup capacitor 71a to the diagnosis circuit 70 for a preset timeperiod after the disconnection. In this case, the time period ofsupplying the energy is a short time since the capacity of the backupcapacitor 71a is small. To obtain a long time continuation of the energysupply, a large capacity must be used. Further, the disconnection alsostops the power supply to the DC power source 64.

Thus, in the air bag system, the energy stored in the backup capacitor71a is used for continuing the diagnosis operation of the diagnosiscircuit 70 after the disconnection, for heating the squib 80 to ignitethe powder and to inflate the air bag. In this respect, it is necessaryto effectively utilize the energy in the backup capacitor 71a and theoutput capacitor 67.

Nevertheless, no measure for effectively utilizing the stored energyunder the power interrupted condition has been taken.

SUMMARY OF THE INVENTION

In view of the above, an object of the present invention is to providean occupant protecting device for use in motor vehicles which cantrigger the ignition device of the occupant protecting device inresponse to a trigger signal which is generated in the event of acollision.

The occupant protecting device must be reliably operated in a heavycollision to destroy the vehicle, a collision of the type in whichinstantaneous damage of the driver is not so large but the damagecontinues for a predetermined period of time, possibly leading to deathof occupants (referred to as a "damage occasioning collision" or inmedium speed collision.

Another object of the present invention is to provide an occupantprotecting device capable of reliably operating a firing circuit in theevent of the damage occasioning collision.

Still another object of the present invention is to provide a powersource circuit suitable for an occupant protecting device for use inmotor vehicles, which is capable of effectively utilizing a limitedelectrical energy stored in an output capacitor even if a power supplyis interrupted by a collision.

To achieve the above object, there is provided an occupant protectingdevice for use in motor vehicles including an acceleration sensor forsensing variation in acceleration of a motor vehicle to detect anoccurrence of a collision, a signal processor for detecting a collisionwaveform from an output signal of the acceleration sensor, a comparatorfor determining whether or not the output signal of the signal processorexceeds a preset level to produce a trigger signal, a latch circuit forlatching the trigger signal from the comparator, and means for operatingan occupant protecter in response to an output signal of the latchcircuit.

The occupant protecting device thus arranged predicts a time rangingfrom an instant that a collision occurs till a driver, for example, isforced to touch a part of his body to the steering, for example,according to the acceleration waveform generated at the collision. Onthe basis of the prediction, the occupant protecting device actuates theoperating unit reliably.

A power source circuit for a vehicle occupant protecting deviceaccording to the present invention is constituted by a DC power sourcefor boosting a battery voltage and supplying the boosted voltage throughan output capacitor to a drive circuit, and a constant voltage circuitfor converting the received voltage into a predetermined voltage and forsupplying the converted voltage to a signal processing circuit. Theoccupant protecting device includes a power feed circuit operating suchthat immediately after the battery voltage decreases below a presetvalue, the power feed circuit operates to feed electric power from theoutput capacitor of the DC power source to the constant voltage circuit.

The power source circuit contains a power feed circuit, which feedselectric power from the output capacitor of a DC power source to aconstant voltage circuit.

The power feed circuit starts to operate immediately after the batteryvoltage decreases below a preset value.

With the provision of the power feed circuit, when the power supply isinterrupted by the disconnection, and the battery voltage decreases tobelow a preset value, the energy is supplied from the output capacitorto the constant voltage circuit. As a result, the energy stored in theoutput capacitor can be effectively utilized.

Since the power feed circuit is operated only when necessary, thedeterioration of its performance by the aging is negligible. In thisrespect, the power source circuit of the embodiment is reliable.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrated presently preferred embodimentsof the invention and, together with the general description given aboveand the detailed description of the preferred embodiments given below,serve to explain the principles of the invention. In the accompanyingdrawings:

FIG. 1 is a block diagram showing a conventional occupant protectingdevice for use in motor vehicles;

FIG. 2 is a block diagram showing a first embodiment of an occupantprotecting device according to the present invention;

FIG. 3(A) is a waveform diagram showing variation in the output of anacceleration sensor and FIG. 3(B) is also a waveform diagram showingdisplacement of a driver's head;

FIG. 4 is a detailed circuit diagram showing an example of the occupantprotecting device according to the present invention;

FIG. 5 is a block diagram showing a second embodiment of an occupantprotecting device for use in motor vehicles according to the presentinvention;

FIG. 6 is a diagram showing a variety of outputs from circuit componentsof the device shown in FIG. 5;

FIG. 7 is a block diagram shown in a third embodiment of the occupantprotecting device according to the present invention;

FIG. 8 is a circuit diagram showing a conventional power source circuitfor an igniting device of a vehicle occupant protecting device;

FIG. 9 is a circuit diagram showing a first embodiment of a power sourcecircuit according to the present invention;

FIG. 10 is a circuit diagram showing a second embodiment of a powersource circuit according to the present invention;

FIG. 11 is a block diagram showing an igniting circuit for use in anoccupant protecting device according to the present invention;

FIGS. 12(a) and 12(c) show waveforms of the output voltage of anauxiliary power source;

FIGS. 12(b) and 12(d) show waveforms of a control signal S correspondingto a power supply time;

FIG. 13 is a circuit diagram showing an example of a diagnosis circuitfor an occupant protecting device; and

FIG. 14 is a circuit diagram showing an embodiment of a diagnosiscircuit for an occupant protecting device according to the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 is a block diagram showing a first embodiment of an occupantprotecting device according to the present invention. In FIG. 2, thesame or equivalent components as those in FIG. 1 bear the same referencenumerals.

In FIG. 2, an acceleration sensor 1 senses variation in an accelerationof a motor vehicle in the event of a collision with another vehicle, forexample, so as to produce an analog signal which represents a conditionof the collision. A first incomplete integrating circuit 2a, having atime constant T1, receives the analog signal from the accelerationsensor 1 to be integrated therein. An second incomplete integratingcircuit 2b, which has the same function as the first incompleteintegrating circuit 2a, incompletely integrates again the output signalfrom the first incomplete integrating circuit 2a. A time constant T2 ofthe second incomplete integrating circuit 2b is equal to the timeconstant T1 of the first incomplete integrating circuit 2a. A firstcoefficient circuit 6, which includes a first attenuator, adds a firstcoefficient to the output signal of the acceleration sensor 1. A secondcoefficient circuit 7 includes a second attenuator having an attenuationfactor of K. The second coefficient circuit 7 adds a second coefficientto the output signal of the first incomplete integrating circuit 2a. Theattenuation factor of the first coefficient circuit 6 is K² ×1/2 where Kis the attenuation factor of the second coefficient circuit 7. Theattenuation factor K is equal to a time t_(d) from an instant that anigniting current is allowed to flow through an igniting device 5 tillthe inflation of an air bag is completed. An adder 8 adds together theoutput signals from second incomplete integrating circuit 2b, the firstcoefficient circuit 6, and the second coefficient circuit 7 to producethe addition result to the comparator 3. The first and second incompleteintegrating circuits 2a and 2b, the first and second coefficientcircuits 6 and 7, and the adder 8 constitute a signal processor. A latchcircuit 9 latches a trigger signal output from the comparator 3. Theoutput signal of the latch circuit 9 ignites the igniting device 5 tooperate an air bag and/or a preloader, for example.

An operation of the occupant protecting device will be describedhereinafter. Various accelerations exert on a motor vehicle when itruns. It is now assumed that a vehicle running at a constant velocity Vocollides with another vehicle, for example, and an acceleration a(t)exerts on the vehicle in the forward/backward direction of the runningvehicle as shown in FIG. 3(A). When the acceleration in theforward/backward direction thereof is sensed by the acceleration sensor1, the head of a driver, for example, is forcibly moved forward with arush, at a constant velocity Vo and the acceleration a(t) also exerts onthe driver. The head moves forward at a velocity relative to thevehicle, viz., a velocity V(t) (=∫a(t)dt). The acceleration output a(t)from the acceleration sensor 1 is integrated by the first incompleteintegrating circuit 2a. With the movement of the head, the head isdisplaced formed by x(t) (=∫V(t)dt). The displacement starts with aninitial position and progresses with time. The initial position is aposition of the head immediately before the collision occurs. Thedisplacement x(t) is calculated by integrating the output signal of thefirst incomplete integrating circuit 2a in the second incompleteintegrating circuit 2b. A quantity of the displacement x(t) of thedriver's head in real time is calculated. Then, the second coefficientcircuit 7 weights the output V(t) of the first incomplete integratingcircuit 2a by td, V(t)×td. That is, a quantity of the displacement for aminute duration of time td is calculated. The acceleration a(t) outputfrom the acceleration sensor 1 is weighted by 1/2t² d in the firstcoefficient circuit 6 to 1/2a(t)×t_(d). That is, a quantity of thedisplacement for a minute duration of time td is calculated. Thoseoutputs are added together, x(t)+V(t)×t_(d) +1/2a(t)×t² _(d).

This corresponds to a predictive value x(t+t_(d)) representing theposition of the driver's head after the time td elapses from the presenttime point. The predictive value x(t+t_(d)) is applied to the comparator3. When the predictive value x(t+t_(d)) exceeds a threshold value x at atime point where the position of the head has been displaced from theinitial position "0" (see FIG. 3(B)), viz., at a time point t1, thecomparator produces a trigger signal which in turn is latched in thelatch circuit 9. The output signal output from the latch circuit 9supplies an igniting current to the igniting device 5. As a consequence,the air bag is inflated thereby protecting the driver. As seen from FIG.3(B), if the position to commence the inflation of the air bag isselected at the position located at a distance X apart from the initialposition 0, the air bag starts inflating at time point t1, viz., time tdearlier than time point t2 where the head actually reaches the positionx.

FIG. 4 is a schematic diagram showing an example of a specific circuitof the occupant protecting device thus constructed and operated. In thefigure, the same reference numerals designate the same or equivalentcircuit components, respectively. The signal is inverted in polarityafter passing the first incomplete integrating circuit 2a. Before theoutput signal of the first incomplete integrating circuit 2a is input tothe second incomplete integrating circuit 2b, the polarity of the outputsignal must be inverted again. To this end, a polarity inverting circuit10 is provided at the prestage of the second incomplete integratingcircuit.

In FIG. 4, the second coefficient circuit 7 is not illustrated since thecoefficient of it is set to 1.

As described above, the occupant protecting device according to thefirst embodiment of the invention comprises an acceleration sensor forsensing an acceleration signal of a vehicle in the event of collisionwith another vehicle or the like, a signal processor for detecting acollision waveform from an acceleration signal output from theacceleration sensor, a comparator for determining whether or not theoutput signal of the signal processor exceeds a preset level to producea trigger signal when it exceeds the preset level, a latch circuit forlatching the trigger signal from the comparator, and a firing circuitfor operating an occupant protecting unit in response to a drive signalfrom the latch circuit.

With such an arrangement of the invention, a trigger signal generated bythe comparator can reliably operate the firing circuit.

An occupant protecting device according to a second embodiment of thepresent invention will be described with reference to FIGS. 5 and 6.

FIG. 5 is a block diagram showing a second embodiment of an occupantprotecting device for use in motor vehicles according to the presentinvention. In FIG. 5, an acceleration sensor 1 includes a piezoelectricceramic element having a pre-amplifier, a filter, and the like. Alow-pass filter (LPF) 14 of 40 Hz receives an acceleration signal outputfrom the acceleration sensor 1. A switch 15 functions to connect anddisconnect the output path of the LPF 14. A first incomplete integratingcircuit 2a which integrates the output signal of the LPF 14 is anintegrating circuit for velocity calculation. A second incompleteintegrating circuit 2b which is coupled to the output of the integratingcircuit 2a, is an integrating circuit for displacement calculation. Afirst coefficient circuit 6 containing a first attenuator, adds a firstcoefficient to the output signal of the acceleration sensor 1 whichcomes through the switch 15. A second coefficient circuit 7 containing asecond attenuator, adds a second coefficient to the output signal of theintegrating circuit 2a. An adder 8 adds together the output signals ofthe second integrating circuit 2b, the first coefficient circuit 6, andthe second coefficient circuit 7. A comparator 3 which receives theoutput signal of the adder 8, produces a signal of high level when thereceived signal exceeds a threshold level. The switch 15, firstintegrating circuit 2a, second integrating circuit 2b, first coefficientcircuit 6, second coefficient circuit 7, adder 8, and comparator 3constitute a first extracter 101. The first extracter 101 functions toextract a first acceleration component which exerts on the occupantsactually.

A first comparator 22 operates such that its output goes high when afirst acceleration increases to exceed 1 G, for example. A secondcomparator 23 operates such that its output goes high when a secondacceleration decreases to exceed 0.5 G, for example. A third comparator24 operates such that its output goes high when a third accelerationincreases to exceed 4 G, for example. A fourth comparator 25 operatessuch that its output goes high when a fourth acceleration increases toexceed 10 G, for example. A first timer 26 is actuated when the outputsignal of the first comparator 22 goes high and the timer 26 keeps itsoutput in a high level state during a first timer period T1ms, A secondtimer 27 is actuated when the output signal of the third comparator 24goes high, and the timer 27 keeps its output in a high level stateduring a second timer period T2ms. An OR gate 28 receives both outputsignals of the first and second timers 27 and 28. An OR gate 29 receivesthe output signals of the second comparator 23 and the OR gate 28. Aset/reset flip-flop (FF) 30 receives at the set terminal the outputsignal of the first comparator 22 and at the reset terminal the outputsignal of the OR gate 29. The output signal of the FF 30 closes theintegrator switch 15 to reset the first integrating circuit 2a for thevelocity-calculation, the second integrating circuit 2b for thedisplacement-calculation, and other integrating circuits. The first tothird comparators 22 to 24, the first and second timers 26 and 27, theOR gates 28 and 29, and the FF 30 constitute a second extracter 102which functions to extract a second acceleration component representingan occurrence of vehicle destruction by collision only.

A set/reset flip-flop (FF) 31 operates such that when it receives theoutput signal from the fourth comparator 25, its output Q goes high, andwhen it receives the output signal from the OR gate 28, it is reset. Aband-pass filter (BPF) 32 of 30 to 200 Hz, for example, receives theacceleration signal from the acceleration sensor 1. A full-waverectifier 33 rectifies the output signal of the BPF 32. An integratingcircuit 34 integrates the output signal of the full-wave rectifier 33. Acomparator 35 produces a high level signal when the output signal of theintegrating circuit 34 exceeds a threshold value. A set/reset flip-flop(FF) 36 is set by the high level signal from the comparator 35, and isreset by the high level signal from the OR gate 28. An OR gate 37receives both output signals from the FFs 31 and 36. The fourthcomparator 25, the FFs 31 and 36, the BPF 32, the full-wave rectifier33, the integrating circuit 34, the comparator 35, and the OR gate 37constitute a third extracter 103. The third extracter 103 extracts achange of the acceleration upon occurrence of dangerous collisionresulting in actual damage to the occupants. An AND gate 38 produces asignal to fire the ignition device 5 when it receives both high levelsignals from the comparator 3 of the first extracter 101 and the OR gate37 of the third extracter 103.

The operation of the occupant protecting device will be described.During the running of a motor vehicle, various accelerations exert onthe acceleration sensor 1. When the vehicle runs at a constant speed,the output Q of the FF 30 is in low level. Accordingly, the switch 15 isin an off state, and the first and second incomplete integratingcircuits 2a and 2b are both in a reset state.

Under this condition, it is assumed that a light collision resulting inno occurrence of significant damage occurs. In the light collision, asshown in FIG. 6, the G wave (denoted as (1)) varies in such a mannerthat it increases to reach a peak (<4 G), and then decreases toward 0 G.At a time point where the G wave (1) exceeds 1 G, the switch 15 isturned on to operate the first timer 26 for a preset period of time.Within this period, if the G wave does not exceed 10 G as indicated by adotted line in the G wave (1), that is, if no heavy collision which willresult in the occurrence of vehicle destruction, occur, the FF 30 isreset, so that the switch 15 and the first and second incompleteintegrating circuits 2a and 2b are reset.

Let us consider another collision where the G wave varies as indicatedby (2). As shown in FIG. 6, the G wave (2) varies in such a manner thatit increases to reach a peak (<4 G) and decreases toward 0 G, and thengreatly increases toward 10 G. In this collision, at a time point wherethe G wave (2) exceeds 1 G, the first timer 26 starts to operate. Beforethe time-up of the first timer 26, the second timer 27 starts tooperate. Accordingly, the timer period of the first timer 26 iselongated twice as long as the equal timer period of time. During theelongated period, if no heavy collision of more than 10 G occurs asindicated by a dotted line (2) in FIG. 6, the switch 15 is turned off,so that no igniting device is fired as is similar to the light collisionmentioned above.

Assuming a heavy collision where the G wave varies as indicated by (3)in FIG. 6, the G wave (3) varies in a manner that it increases to reacha peak (<4 G) and decreases toward 0 G, and then greatly increases andexceeds 10 G. In this collision, within the operating time of the firsttimer 26 or the operating time of the second timer 27 which follows theoperation of the first timer 26, both FF 31 or FF 36 is set, as a resultof which the output of the FF 31 or 36 goes high (H or J in FIG. 6). Theoutput of the OR gate 37 which receives this high level signal, goeshigh (M in FIG. 6). The output signal (O in FIG. 6) from the comparator3 of the first extracting means 101, which has been in high level, andthe high level signal output from the OR gate 37 enable the AND gate 38to produce a high level signal in turn. The high level signal outputfrom the AND gate 38 actuates the igniting device 5 to inflate at leastone of air bags, for example.

In the second embodiment of FIG. 5, the igniting device 5 is fired byeither the high level signal output from the FF 31 in the thirdextracter 103, which is set when the serious collision occurs or thehigh level signal output from the FF 36 in the third extracter 103,which is set when the damage occasioning collision occurs. In this case,however, these high level signals occurs at the substantially same time,and therefore the second extracter 102 may be omitted for simplifyingthe circuit construction.

As described above, the occupant protecting device for use in motorvehicles of the present invention, which determines, when a collisionoccurs, whether or not a serious collision occurs according to anacceleration signal output from an acceleration sensor. Based on thedetermination, the device actuates an igniting device of the occupantprotecting device. The device includes a plurality of extracters forextracting, from the acceleration signal of the acceleration sensor, afirst acceleration component which exerts on occupants actually, asecond acceleration component showing only the vehicle destruction, andothers indicating a change of the acceleration, so that the ignitingdevice is operated in response to the extracting result from eachextracter.

With such an arrangement, when the serious collision causing vehicledestruction or the damage occasioning collision occurs, the occupantprotecting device is reliably operated. In the other collisions than theabove, undesired occupant protecting operation which may requirereplacement of the device is prevented positively accomplished.

A third embodiment of the occupant protecting device according to thepresent invention will be described with reference to FIGS. 5 and 7. InFIG. 7, the same or equivalent circuit components bear the samereference numerals in FIG. 5.

In FIG. 7, first to fourth comparators 22 to 25, first and second timers26 and 27, OR gates 28 and 29, and FFs 30 and 31 constitute a controlcircuit 102 for controlling the first and second incomplete integratingcircuits 2a and 2b.

A low-pass filter (LPF) 32' of 30 Hz, for example, receives anacceleration signal from the acceleration sensor 1. A subtractor 39calculates subtraction of the outputs of the LPFs 14 and 32' to producea limited signal having a frequency within a predetermined frequencyband. A full-wave rectifier 33 rectifies the output signal of thesubtractor 39. An integrating circuit 34 integrates the output signal ofthe full-wave rectifier 33, and is reset by the output signal of the FF30. A comparator 35 produces a high level signal when the output signalof the integrating circuit 34 exceeds a threshold value. A set/resetflip-flop (FF) 36 is set by the high level signal from the thresholdcircuit 35, and is reset by the high level signal from the OR gate 28.The LPF 32', subtractor 39, full-wave rectifier 33, integrating circuit34, comparator 35 and FF 36 constitute a medium speed detector 103. AnOR gate 37 receives the output signals of the FFs 31 and 36. An AND gate38 produces a signal to fire the igniting device 5 when it receives highlevel signals from the comparator 3 of the first extracter 101 and theOR gate 37 of the medium speed detector 103.

The operation of the occupant protecting device will be described. Whenthe vehicle runs at a constant speed, an output C at the terminal Q ofthe FF 30 is in low level. Accordingly, the switch 15 is in an offstate, and the incomplete integrating circuits 2a and 2b are both in areset state.

Under this condition, it is assumed that a light collision occurs. Inthe light collision, as shown in FIG. 6, the G wave (denoted as (1))varies in a manner that it increases to reach a peak (<4 G), and thendecreases toward 0 G. At a time point where the G wave (1) exceeds 1 G,the switch 15 is turned on to operate the first timer 26 for a presetperiod of time. Within this period, if no heavy collision exceeding 10 Goccurs, the FF 30 is reset, so that the switch 15 and the first andsecond incomplete integrating circuits 2a and 2b are reset.

Let us consider another collision where the G wave varies as indicatedby (2) in FIG. 6. As shown in FIG. 6, the G wave (2) varies in such amanner that it increases to reach a peak (<4 G) and decreases toward 0G, and then greatly increases toward 10 G. In this collision, at a timepoint where the G wave (2) exceeds 1 G, the first timer 26 starts tooperate. Before the time-up of the first timer 26, the second timer 27starts to operate. Accordingly, the timer time of the first timer 26 iselongated by the equal period of time. During the elongated period, theheavy collision of more than 10 G does not occur. Accordingly, theswitch 15 is turned off, so that no firing operation to the ignitingdevice 5 is carried out. In other words, the occupant protecting deviceaccording to the present invention may not respond to the lightcollision referred to above.

Let us consider a heavy collision where the G wave varies as indicatedby (3). As shown, the G wave (3) varies in such a manner that itincreases to reach a peak (<4 G) and decreases toward 0 G, and thengreatly increases and exceeds 10 G. In this collision, within theoperating time of the first timer 26 or the operating time of the secondtimer 27 which follows the operation of the first timer 26, the FF 31 isset. Or the FF 36 of the medium speed detector 103 is set by a mediumspeed collision. As a result, the output of the FF 31 or 36 goes high (Hor J in FIG. 6). The output of the OR gate 37, which receives this highlevel signal, goes high (M in FIG. 6). The output signal (O in FIG. 6)from the comparator 3 of the first extracter 101, which has been in highlevel, and the high level signal output from the OR gate 37 enable theAND gate 38, which in turn produces a high level signal. The high levelsignal output from the AND gate 38 operates the igniting device 5, toinflate an air bag, for example.

In the third embodiment of FIG. 7, the igniting device 5 is driven bythe high level signal output from the FF 31 in the control circuit 102,which is set when the serious collision occurs or the high level signaloutput from the FF 36 in the medium speed collision detector 103, whichis set when the medium speed collision occurs. As explained above, thesehigh level signals occur at the substantially same time, and thereforethe control circuit 102 may be omitted for simplifying the circuitconstruction.

As described above, the occupant protecting device in use with motorvehicles determines, when a collision occurs, whether or not a seriouscollision occurs according to an acceleration signal output from anacceleration sensor, and operates an occupant protecting device bodyproper in response to the acceleration signal. The occupant protectingdevice comprises a third extracter operating in the following way. Theoutput signal of the acceleration sensor is filtered out into a signalof a frequency within a predetermined frequency band. The filteredsignal, after full-wave rectified, is integrated for a preset time. Whena value of the integration exceeds a preset value, the collision isdetermined as a medium speed collision. On the basis of thedetermination result, a signal is produced to operate the ignitingdevice of the occupant protecting device. With such an arrangement, theoccupant protecting device can reliably operate the occupant protectingdevice body proper even when a medium speed collision occurs.

FIG. 9 is a circuit diagram showing a first embodiment of a power sourcecircuit according to the present invention. The power source circuit isto be incorporated into an air bag system for motor vehicles. In FIG. 9,the same or equivalent circuit components are designated by the samereference numerals as those shown in FIG. 8.

The diagnosis circuit 70 and the storage portion 72 receive an electricpower from the diagnosis power source 71. The input terminal of aswitching regulator 84 is connected to the positive terminal of theoutput capacitor 67. The switching regulator 84 includes an on/offcontrol terminal.

The on/off control terminal is electrically connected to the negativeterminal 62a of the ignition switch 62. With this connection, a batteryvoltage is supplied from the battery 61 to the on/off control terminal,through the ignition switch 62.

When the battery voltage is applied to the on/off control terminal, theswitching regulator 84 stops its operation.

When the voltage applied to the on/off control terminal becomes zero invalue, the switching regulator 84 starts to operate and to produce agiven electric power.

The output terminal of the switching regulator 84 is connected to theanode of a diode 85.

The cathode of the diode 85 is connected to the input terminal of thediagnosis power source 71.

The switching regulator 84 converts the electrical energy of the outputcapacitor 67 into a DC power, which in turn is supplied through thediode 85 to the diagnosis power source 71.

If a heavy collision occurs and the ignition switch 62 is disconnectedfrom the controller 63 at a position denoted as X, the battery voltageof the car-carried battery 61 abruptly decreases to zero.

At this time, the backup capacitor 71a which is connected to the inputterminal, backs up the diagnosis circuit 70 and the storage portion 72,in the case of the conventional power source circuit as already stated.

The back-up time T1 is mathematically expressed

    T1=-C.sub.1 ·Rln (V.sub.3 /V.sub.1)               (1)

where V₃ : minimum input voltage to the diagnosis power source 71;

V₁ : voltage across the backup capacitor 71a at the start of the back-upoperation;

R: combined impedance when viewing the diagnosis power source 71 fromthe backup capacitor 71a; and

C₁ : capacitance of the back-up capacitor.

On the other hand, in this embodiment, the switching regulator 84, whichhas been at standstill, starts its operation at the instant that theignition power supply is stopped by the disconnection at the position X,to generate an electric power using the electrical energy stored in theoutput capacitor 67 to be supplied to the input terminal of thediagnosis power source 71.

In this embodiment, the back-up time T₁ when the circuits are backed upby the output capacitor 67 is mathematically expressed:

    T.sub.2 =-C.sub.2 ·R.sub.1 ln (V.sub.6 /V.sub.4)  (2)

where V₆ : minimum input voltage to the switching regulator 84;

C₂ : capacitance of the output capacitor 67;

V₄ : voltage across the output capacitor 67 at the start of the back-upoperation; and

R₁ : combined impedance when seeing the switching regulator 84 from theoutput capacitor 67.

The back-up time T for the diagnosis circuit 70 and the storage portion72 is the sum of the back-up time T₂ by the output capacitor 67 and theback-up time T₁ by the backup capacitor 71a. Hence, the back-up time Tis expressed as follows:

    T=T.sub.1 +T.sub.2 =-C.sub.1 ·Rln(V.sub.6 /V.sub.4) (3)

The switching regulator 84 may be substituted by a so-called seriesregulator, such as a 3-terminal regulator.

Further, the switching regulator 84 may be omitted. In this case, theplus terminal of the output capacitor 67 is connected to the diode andto the diagnosis power source 71.

In the case where the ignition switch 62 is disconnected from thecontroller 63, the battery voltage of the car-carried battery 61abruptly decreases to zero, this embodiment operates in the followingway. The switching regulator 84 detects this state and starts itsoperation to produce an electrical power using the energy stored in theoutput capacitor 67. The electrical power is supplied to the inputterminal of the diagnosis power source 71.

The diagnosis power source 71 generates an electrical power using thepower supplied from the switching regulator 84 and the power stored inthe backup capacitor 71a, and supplies it to the diagnosis circuit 70.

The switching regulator 84 operates only when the disconnection occurs.Therefore, the deterioration of the switching regulator 84 inperformance by aging is minimized, and hence the power source circuitobtained is remarkably reliable.

As described above, in the power source circuit of this embodiment, thepower feed circuit operates to feed the power from the output capacitor,which is contained in the DC power source, to the constant voltagecircuit, only when the disconnection occurs.

FIG. 10 is also a circuit diagram showing a second embodiment of a powersource circuit according to the present invention. As is apparent fromFIG. 10, a diode 86 is additionally provided to the circuit shown inFIG. 9. The diode 86 functions to feed back the output power of thediagnosis power source 71 to the input of the DC power source 64 forinflating the air bag.

The anode of the diode 42 is connected to the output terminal of thediagnosis power source 71 whereas the cathode of the diode 42 isconnected to the DC power source 64.

With provision of the diode, when the disconnection occurs, part of thelimited electrical energy stored in the capacitor of the diagnosis powersource 71 is returned through the diode 42 to the DC power source 64,which then supplies the received power to the combinations ofacceleration switches and resistors 74a and 74b to 79a and 79b, and tothe igniting or firing controller 80A.

Since the power source circuit is thus arranged, it can reliably operatethe air bag system when the electric power drops by the disconnectionwhen the collision occurs.

In the power source circuit of the second embodiment, when thedisconnection trouble takes place, the output capacitor in the DC powersource feeds the power to the constant voltage circuit. At the sametime, part of the output voltage of the constant voltage circuit is fedback to the input of the DC power source. Therefore, the effectiveutilization of the electrical energy stored in the output capacitor isrealized.

In the power source circuit of FIG. 9 or 10, the voltage of the battery61 is boosted by the DC--DC converter 65, to gain a stabilized highvoltage. The high voltage is stored in the auxiliary power source, oroutput capacitor 67. Utilizing the stored high voltage, the power issupplied to the squib 80, to fire a powder to inflate the air bag.Because of the construction of the power source circuit, when thevoltage of the auxiliary power source is lower than a predeterminedvoltage or it varies, it fails to operate the air bag system.

To avoid this, the DC--DC converter 65 which boosts the battery voltageto generate a high voltage and then supplies the power to the auxiliarypower source, must be reliable and hence is expensive.

To solve the above problem, an igniting circuit in which an ordinaryboost circuit simple in construction and low in cost is employed inplace of the DC--DC converter, in order to boost the battery voltage togenerate a high voltage and to supply the power to the auxiliary powersource.

To be more specific, the igniting circuit uses an inexpensive and simpleboosting circuit instead of the expensive DC--DC converter 65 includinga first converter for converting the output power of a DC power sourceinto an AC power, a second converter for converting the AC power fromthe first converter into a DC power and a third converter for convertingthe DC voltage from the second converter into a voltage several times aslarge as the received DC voltage.

The igniting circuit further includes signal time-width varying unit forvarying the output time width of a signal for driving the occupantprotecting device according to the voltage value of the auxiliary powersource.

With such an arrangement, when the output voltage of the boostingcircuit varies and the voltage value of the auxiliary power sourcevaries, the output time width of the signal for driving the ignitingcircuit varies according to the varying voltage value.

The igniting circuit, although it is simple and inexpensive, is able tosupply the electric power enough to drive the occupant protectingdevice.

FIG. 11 is a block diagram showing an igniting circuit for use in anoccupant protecting device according to the present invention. In FIG.11, a car-carried battery 101 is a main power source in the occupantprotecting device for a motor vehicle. A boost circuit 102 boosts thevoltage of the battery 101. The boost circuit 102 which forms the abovedescribed first to third converters, is constituted by a voltagemultiplying rectifier circuit including a transistor bridge, capacitorsand diodes. The boost circuit converts a DC power to an AC power. Anauxiliary power source 103 is an auxiliary power source having acapacitor, which supplies an igniting power to an igniting device 107 tobe given later when a collision accident occurs.

A control circuit 104 determines whether or not the ignition power issupplied to the ignition device 107 on the basis of the output signalfrom an acceleration sensor 105, and applies a pulsasive control signalS to a transistor 106.

The control circuit 104 includes a pulse-width varying circuit 104a.

The pulse-width varying circuit 104a detects the output voltage of theauxiliary power source 103 and varies the pulse width of the controlsignal S according to the detected voltage. In response to the controlsignal S of the control circuit 104, the transistor 106 is operated tosupply the power from the auxiliary power source 103 to the ignitiondevice 107.

The ignition device 107 operates in response to the control signal Sfrom the control circuit 104 to inflate the air bag, mounted on thesteering, for example.

When a motor vehicle runs normally, the output voltage of the battery101 is boosted by the boost circuit 102 to be supplied to the auxiliarypower source 103.

The boost circuit 102 boosts the battery voltage integer times andsupplies it to the auxiliary power source 103. The boosted voltage isstill instable.

Accordingly, the output voltage of the auxiliary power source 103depends on the output voltage of the battery 101. When the battery 101is discharged and its output decreases, the output voltage Vc of theauxiliary power source 103 also decreases.

The pulse-width varying circuit 104a of the control circuit 104 monitorsthe output voltage Vc of the auxiliary power source 103.

When the vehicle collides with another vehicle, for example, the controlcircuit 104 produces a control signal S to the transistor 106 on thebasis of the voltage from the acceleration sensor 105.

The transistor 106 is turned on, so that the supply of an ignition powerfrom the auxiliary power source 103 to the ignition device 107 starts.

The pulse-width varying circuit 104a determines the time necessary forsupplying the power to drive the igniting device 107 using the outputvoltage Vc of the auxiliary power source 103, and stops the supply ofthe control signal S when the power to drive the igniting device 107 hasbeen supplied.

Accordingly, the transistor 106 is turned off, to stop the power supplyfrom the auxiliary power source to the ignition device 107.

The method to determine the power supply time will be described usingtwo cases; a first case where the output voltage of the auxiliary powersource 103 at the start of the collision is Vca in FIG. 12(a), and asecond case where the output voltage is Vcb where Vca>Vcb as shown inFIG. 12(b).

The electric energy E necessary for firing the igniting device 107 ispreviously calculated in connection with the igniting device 107.

The output voltage Vc(t) of the auxiliary power source 103 varies asshown in FIGS. 12(a) and 12(c), depending on the initial value at thestart of the power supply to the igniting device 107, the capacitance ofthe auxiliary power source 103, and the load including the ignitingdevice 107. The output voltage Vc(t) is sampled every preset period oftime, starting from a time point where the control signal S is output.The sampled values are accumulatively calculated. A time taken for thecalculated value to reach a value corresponding to the electric energy E(indicated by the shaded areas in the graphs) is the power supply time.

At the time point where the control signal S is output, the outputvoltage Vc of the auxiliary power source 103 shown in FIG. 12(a) isequal to the output voltage Vc in the case of FIG. 12(c). Accordingly,the power supply time ta in the case of FIG. 12(a) is smaller than thattb of FIG. 12(b).

FIGS. 12(b) and 12(d) show waveforms of the control signal Scorresponding to the power supply time.

In the embodiment as mentioned above, the output voltage Vc(t) issampled every preset period of time, and the sampled values areaccumulatively calculated. A time taken for the calculated value toreach a value corresponding to the electric energy E is used as thepower supply time. The embodiment may be modified as follows. The outputvoltage Vc(t) of the auxiliary power source 103 is integrated by anintegrator. When the output value of the integrator reaches the valuecorresponding to the electric energy E, the supply of the control signalS is stopped.

In another modification, a microcomputer is used. In this case, theoutput voltage Vc(t) of the auxiliary power source 103 is digitizedevery preset period by an A/D converter. The digital signals obtainedare accumulated. When the accumulated value reaches the valuecorresponding to the electric energy E, the supply of the control signalS is stopped.

The discharge characteristic of the auxiliary power source 103 dependson the initial value at the start of the power supply to the ignitingdevice 107, the capacitance of the auxiliary power source 103, and theload including the igniting device 107.

In this case, the capacitance of the auxiliary power source 103, and theload including the igniting device 107 are constant. The power supplytime satisfying the electric energy E necessary for driving the ignitingdevice 107 is previously calculated for the initial voltage value of theauxiliary power source 103. A time table for defining the power supplytime vs. the initial voltage time of the auxiliary power source 103 isprepared and stored into a memory.

The microcomputer or CPU fetches digital signals representing the outputvoltage Vc(t) of the auxiliary power source 103, from the A/D converterat the start of the power supply to the igniting device 107. It refersto the data table using the fetched output voltage (initial voltagevalue) of the auxiliary power source 103, and reads the correspondingpower supply time from the data table. On the basis of the readout data,the CPU sets the power supply time and turns off the control signal S ata time point where the power supply time terminates. In this way, thepower supply to the ignition device 107 is controlled.

As described above, the output time width of the signal for driving theoccupant protecting device is varied in accordance with the voltagevalue of the auxiliary power source. Accordingly, a simple andinexpensive boost circuit may be acceptable. Further, even if the outputvoltage of the auxiliary power source is instable, the ignition devicecan be reliably driven.

FIG. 13 is a circuit diagram showing an example of a diagnosis circuitfor an occupant protecting device. The diagnosis circuit determineswhether the igniting circuit is normal or abnormal by feeding a minutecurrent into the igniting circuit.

In FIG. 13, reference numeral 61 designates a car-carried battery; 62,an ignition switch; and 64, a DC power source 64 for ignition, which isprovided with a boosting DC--DC converter.

A high-pass filter 111, an integrator 112, a comparator 113 and anamplifier 114 constitute an operation determining circuit 110 in thediagnosis circuit. The operation determining circuit 110 receives anoutput from the acceleration sensor 1.

A voltage dividing resistor 115 for diagnosis, an acceleration switch74a and a resistor 74b coupled in parallel to the switch 74a, and ansquib 80 for firing the powder to inflate the air bag are furtherprovided as shown in FIG. 13.

A constant current circuit 116 operates to allow a constant current of100 mA to flow thorough the squib 80. A switch circuit 116a is providedto actuate the constant current circuit 116. A differential amplifier117 is connected in such a manner that the inverting input terminalthereof is connected through a resistor to the power source terminal ofthe ignition circuit or squib 80, and the non-inverting input terminalis connected through a resistor to the ground terminal of the squib 80.

The differential amplifier 117 detects a voltage appearing across theterminals of the squib 80 and amplifies the detected voltage to besupplied to the following comparator 118. The comparator 118 has anon-inverting input terminal connected to the plus terminal of areference voltage source 119 for producing a reference voltage Vref. Theinverting input terminal of the comparator 118 is connected to theoutput terminal of the differential amplifier 117.

In the diagnosis circuit for the occupant protecting device, arelatively large current of 100 mA for instance is fed to the squib 80in order that an offset of the signal output from the differentialamplifier 117 is set small, and that the differential amplifier 117 canproduce a signal large enough to discriminate a normal state of thesquib 80 from a failure state of disconnection or just-beforedisconnection.

The operation of the diagnosis circuit thus arranged will be described.

When an impact acceleration exerts on a vehicle body in the event of acollision, the acceleration sensor 1 and the acceleration switch 74acooperate to detect variation in such an acceleration.

As a result, the acceleration switch 74a is turned on and an electricalsignal representing the acceleration output from the acceleration sensor1 is applied to the operation determining circuit 110.

The signal is applied to the base terminal of the output transistor ofthe amplifier 114, through the high-pass filter 111, integrator 112 andcomparator 113, so that the output transistor of the amplifier 114 isturned on to thereby cause a DC power source 64 to supply a currentlarge enough to actuate the squib 80.

Consequently, the squib 80 fires the powder to inflate the air bag.

When no acceleration is applied to the vehicle body, the accelerationswitch 74a and the output transistor of the amplifier 114 are in an offstate.

To diagnose the squib 80, the switch circuit 116a is closed. Then, aconstant current of 100 mA is allowed to be fed from the DC power source64 to the squib 80 for the purpose of diagnosis operation. It should benoted that the constant current of 100 mA is not so large to actuate thesquib 80. When the constant current flows through the squib 80, apotential difference appears across the squib 80 depending on acondition of the squib 80 such as normal state, a state of disconnectionor a state of just-before disconnection.

The differential amplifier 117 amplifies this potential difference to beapplied to the comparator 118 where the signal output from thedifferential amplifier 117 is compared with the reference voltage Vrefof the reference voltage source 119. When the former exceeds the latter,the differential amplifier 117 produces a failure signal indicative of afailure of the squib 80.

When there occurs poor connection or disconnection in the squib 80, thepotential difference across the squib 80 is larger than that when thesquib 80 is in a normal state. In this case, the potential differencemay increase up to the power source voltage. As a result, the outputsignal of the comparator 118 is larger, the signal applied to theinverting input terminal of the comparator 118 exceeds the referencevoltage Vref, so that the comparator 118 produces a failure signal.

In the diagnosis circuit of FIG. 13, the current fed to the squib 80 fordiagnosing the squib 80 is a constant current of 100 mA, which cannotactuate the squib 80. However, this current is not so small relative tothe operating current of the squib 80, and thus If the constant currentaccidentally and instantaneously increases, it may be likely to actuatethe squib 80 erroneously.

To solve this problem, modification is possible with using a minutecurrent thereby improving the reliability of the diagnosis circuit.Shown in FIG. 18 is an embodiment of the modification where a minutecurrent is fed to the squib for diagnosing operation.

In FIG. 18 which is a circuit diagram showing the embodiment of theinvention, the same or equivalent circuit components are designated bythe same reference numerals as those in FIG. 17.

A constant current circuit unit 116 which is constituted by first andsecond constant current sources 116b and 116c and a switch circuit 116d,is inserted between the squib 80 and ground.

The first constant current 116b is able to feed a constant current of100 mA. The second constant current 116c is able to feed a constantcurrent of 20 mA.

In response to a switch signal SS which is produced by a control circuit123, the switch circuit 116d is turned to either one of the firstconstant current source 116b or the second constant current 116c tosupply the current of 10 mA or 100 mA for diagnosis to the squib 80alternatively.

A differential amplifier 117 detects a potential difference across thesquib 80 when it is fed with 10 mA or 100 mA.

A first sample/hold circuit 120 samples and holds the output signal ofthe differential amplifier 117 in response to a sample pulse signal SP1from the control circuit 123. A second sample/hold circuit 121 samplesand holds the output signal of the differential amplifier 117 inresponse to a sample pulse signal SP2 therefrom.

The control circuit 123 produces the switch signal SS to be applied tothe switch circuit 116d, the sample pulse signal SP1 to be applied tothe first sample/hold circuit 120, and the sample pulse signal SP2 to beapplied to the second sample/hold circuit 121. The control circuit 123outputs the sample pulse signal SP1 in synchronism with the time periodthat the switch circuit 116d is turned to the first constant current116b by the switch signal SS. The control circuit 123 outputs the samplepulse signal SP2 in synchronism with the time period that the switchcircuit 116d is turned to the second constant current 116c by the switchsignal SS.

A second differential circuit 122 detects a difference between theoutput signals of the differential amplifier 117 which are sampled andheld by the first and second sample/hold circuits 120 and 121, toproduce an amplified output signal indicative of the difference.

The comparator 118 receives at the inverting input terminal theamplified output signal of the second differential circuit 122. On theother hand, the same receives at the non-inverting input terminal thereference voltage Vref from the reference voltage source 119.

The operation of the diagnosis circuit for an occupant protecting devicewill be described.

To diagnose the occupant protecting device, the control circuit 123sends the switch signal SS, and sample pulse signals SP1 and SP2 to theswitch circuit 116d, the first sample/hold circuit 120, and the secondsample/hold circuit 121.

In response to the switch signal SS, the switch circuit 116d is turned,at fixed periods, to the first constant current source 116b or thesecond constant current source 116c, so that either a constant currentof 10 mA or 20 mA is allowed to flow through the squib 80. A voltageappears across the squib 80, which depends on the current flowingthrough the squib 80.

The differential amplifier 117 detects this voltage and applies thedetected voltage to both the first sample/hold circuit 120 and thesecond sample/hold circuit 121.

The first sample/hold circuit 120 samples and holds the output signal ofthe differential amplifier 117 when the switch circuit 116d is turned tothe first constant current source 116b. On the other hand, when theswitch circuit 116d is turned to the second constant current source116c, the second sample/hold circuit 121 samples and holds the outputsignal of the differential amplifier 117.

The output signal of the differential amplifier 117, which has beensampled and held by the first sample/hold circuit 120, represents thevoltage appearing across the squib 80 when the minute current of 10 mAis allowed to flow through the squib 80. On the other hand, the outputsignal of the differential amplifier 117, which has been sampled andheld by the second sample/hold circuit 121, represents the voltageappearing across the squib 80 when the minute current of 20 mA isallowed to flow therethrough. Accordingly, a difference between thesevoltages across the squib are twice as much as the voltage appearingacross the squib when a single constant current source is employed tocause a constant current to flow through the squib.

For this reason, the amplification factor of the differential amplifier117 may be correspondingly reduced, so that the offset contained in theoutput signal of the amplifier is made smaller. Further, due to theusing of the above described difference between the two output signalsof the differential amplifier 117, the offsets contained in those outputsignals are canceled accordingly resulting in improving the reliabilityof the occupant protecting device.

When the squib 80 is normal, the constant current of 10 mA or 20 mAflowing through the squib 80 causes a constant voltage drop across thesquib 80. The comparator 118 determines whether the squib 80 is normalor abnormal on the basis of the constant voltage drop across the squib80.

When a disconnection occurs in the squib 80, the difference between theoutput signals of the differential amplifier 117 which are sampled anheld by the first sample/hold circuit 120 and the second sample/holdcircuit 121, respectively becomes zero. When receiving this signal, thecomparator 118 decides that the squib 80 is abnormal.

In this embodiment, a minute constant current of 10 mA or 20 mA is fedto the squib 80 to cause different voltage drops across the squib 80.The detected voltage drops across the squib are sampled and held by thesample/hold circuits in synchronism with the supply of the differentminute currents to the squib. These different voltage drops are used forthe diagnosis. Since the current fed to the squib 80 is small, there isno possibility of actuating the squib 80 erroneously.

What is claimed is:
 1. A drive device for a vehicle occupant protectingdevice comprising:an auxiliary power source which is charged by a DCpower source; an igniting device; a switching element connected betweena point connecting the DC power source with said auxiliary power sourceand said igniting device; and a control circuit for judging a magnitudeof a collision based on an acceleration signal supplied from anacceleration sensor to provide a trigger signal to said switchingelement thereby electrically connecting said auxiliary power source tosaid ignition device when it is judged that an accident has occurred,said control circuit detecting an output voltage of said auxiliary powersource. wherein said control circuit comprising a pulse-width varyingcircuit for varying a pulse width of the trigger signal in inverseproportion to the output voltage, and wherein a predetermined energynecessary to fire said ignition device is determined by said outputvoltage and said pulse width, and said control circuit varies said pulsewidth to deliver said predetermined energy to said ignition device. 2.The drive device according to claim 1, wherein said control circuitmonitors said output voltage at discrete intervals of time, andaccumulates sampled values, said control signal being terminated whensaid accumulated sample values correspond to said predetermined energy.3. The drive device according to claim 1, wherein said control circuitcomprises an integrator for integrating said output voltage of saidauxiliary power source, wherein said control signal is terminated whenan output value of said integrator reaches a value corresponding to saidpredetermined energy.
 4. The drive device according to claim 1, whereinsaid control circuit comprises a microcomputer for digitizing saidoutput voltage of said auxiliary power source for a preset period oftime to thereby generate digital signals which are accumulated, whereinsaid control signal is terminated when an accumulated value of saiddigital signals reaches a value corresponding to said predeterminedenergy.